Compression ignition engines, such as diesel engines, produce an exhaust emission that generally contains at least four classes of pollutant that are legislated against by inter-governmental organisations throughout the world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM).
Oxidation catalysts comprising platinum group metals (PGMs) have been used to treat carbon monoxide (CO) and hydrocarbons (HCs), including the volatile organic fraction (VOF) of particulate matter (PM), in exhaust emissions produced by diesel engines. Such catalysts treat carbon monoxide (CO) by oxidising it to carbon dioxide (CO2), and treat hydrocarbons (HCs) by oxidising them to water (H2O) and carbon dioxide (CO2). Some platinum group metals, particularly when supported on a refractory oxide, can also promote the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). It has been found that platinum (Pt) and palladium (Pd) are each able to oxidise carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gas from a compression ignition engine. Palladium is generally cheaper than platinum, but is less active toward CO and HCs (e.g. Pd has a higher light-off temperature for CO and HCs than Pt). Palladium is also more susceptible to poisoning by sulfur in fuel compared to platinum, and may poison the oxidative activity of platinum toward some HCs.
As emissions standards for permissible emission of pollutants from compression ignition engines, particularly vehicular diesel engines, become progressively tightened, there is a need to provide improved exhaust systems that are able to meet these standards and which are cost-effective. To maximise the overall reduction in pollutants produced by a compression ignition engine it is important that the oxidation catalyst works in conjunction with other emissions control devices that form part of the overall exhaust system.
Shortly after start-up of a compression ignition engine the exhaust gas temperature is relatively low. At such temperatures, the oxidation catalyst may be below its effective operating temperature and a significant proportion of hydrocarbons (HCs) in the exhaust gas can pass through the catalyst without being oxidised. To prevent emission of HCs into the atmosphere under such conditions, oxidation catalysts often include a hydrocarbon adsorbent (HCA), to trap HCs at low temperatures and to release the HCs when the oxidation catalyst has reached its effective operating temperature.
It is conventional to locate the hydrocarbon adsorbent (HCA) in a washcoat region of the oxidation catalyst so that it is brought into contact with the inlet exhaust gas before many of the other components of the catalyst (e.g. the HCA is often included in the front zone and/or the topmost layer of the oxidation catalyst). The HCA can then trap HCs at relatively low temperatures before they come into contact with, and potentially block the active sites of, other components of the catalyst (e.g. the PGM). When the HCs are released at higher temperatures, this arrangement can also facilitate contact of the HCs with the PGM component of the oxidation catalyst. This arrangement is generally used when the HCA is a zeolite. The zeolite is segregated from the PGM to minimise migration of PGM to a surface of the zeolite thereby avoiding a loss of oxidative activity. Segregation of the zeolite from the PGM component can be important when the PGM is palladium because silica in the zeolite can poison palladium.